Pellet reclamation process

ABSTRACT

A method of recovering iron and oxide impurities from steel furnace dust in which the dust first is mixed with finely divided coke and then this mixture is pelletized. The green pellets thus formed are deposited over a layer of burnt pellets on a rotary hearth which successively conveys the pellets first through a drying zone, then through an initial heating zone in which the pellets are gradually raised to a temperature at which the coke starts to burn, then through a decontamination zone in which the pellet temperature is rapidly raised to a degree at which zinc, lead and sulfur impurities vaporize and in which these impurities are carried off and collected as oxides, and finally the pellets are carried through a reoxidation and hardening zone in which the temperature thereof is further increased to a sufficient degree and held for a long enough period of time to permit the growth of grains of an oxide of iron on the surface of the pellets, thus to form hard bonded pellets which are not fused together.

Unite States Patent [191 Holley 51 Sept. 17, 1974 [54] PELLETRECLAMATION PROCESS [76] Inventor: Carl A. Holley, 116 Marian Ave.,

Glenshaw, Pa. 15116 22 Filed: Mar. 24, 1971 21 Appl. No.: 127,748

Related U.S. Application Data [63] Continuation of Ser. No. 768,727,Oct. 18, 1968,

abandoned.

[52] U.S. Cl. 75/3, 75/25 [51] Int. Cl C2lb l/30 [58] Field ofSearch75/3, 4

[56] References Cited UNITED STATES PATENTS 1,930,010 10/1933 Haswell75/25 X 2,040,825 5/1936 Betterton et a1 75/25 2,919,231 12/1959 Donath75/4 3,050,298 8/1962 Hall 263/28 3,149,958 9/1964 Ward 75/5 3,180,7234/1965 McCauley.. 75/3 X 3,264,092 8/1966 Ban 75/3 3,386,816 6/1968English 75/3 3,460,818 8/1969 Greanes et al 75/3 X Primary Examiner-A.B. Curtis Attorney, Agent, or Firm-Shenier & OConnor [57] ABSTRACT Amethod of recovering iron and oxide impurities from steel furnace dustin which the dust first is mixed with finely divided coke and then thismixture is pelletized. The green pellets thus formed are deposited overa layer of burnt pellets on a rotary hearth which successively conveysthe pellets first through a drying zone, then through an initial heatingzone in which the pellets are gradually raised to a temperature at whichthe coke starts to burn, then through a decontamination zone in whichthe pellet temperature is rapidly raised to a degree at which zinc, leadand sulfur impurities vaporize and in which these impurities are carriedoff and collected as oxides, and finally the pellets are carried througha reoxidation and hardening zone in which the temperature thereof isfurther increased to a sufficient degree and held for a long enoughperiod of time to permit the growth of grains of an oxide of iron on thesurface of the pellets, thus to form hard bonded pellets which are notfused together.

7 Claims, 5 Drawing Figures PATENTEDSEP! 7 1914 SHEU 2 OF 5 W QUX HTTORNEYS PATENIEDSEPI 7:914

SHEET 5 0F 5 INVENTOR C r/ H. HO/ky BY QTTOP/VEYS PEILLIET RECLAMATIONPROCESS This application is a continuation of my copending applicationSer. No. 768,727 fied Oct. I8, 1968, now abandoned.

BACKGROUND OF THE INVENTION Both the basic oxygen furnace (BOF) and theopen hearth (OH) processes of making steel employ large amounts ofscrap, much of which is galvanized. In the course of each process, thereis produced a dust containing oxides of iron as well as oxides ofimpurities such as zinc, lead and sulfur. In the prior art, this dustnot only is waste, but also creates a problem, since it must be disposedof if contamination of the surrounding atmosphere is to be avoided.Particularly, the dust contains a relatively large percentage of zincferrite, zinc oxide and sulfur. Because of the presence of theseimpurities, attempts to reclaim it directly for re-use have not provedto be practical.

As an alternative to direct recovery, it has heretofore been proposedthat the dust first be pelletized and then subjected to a very hightemperature for a short period of time to sinter the pellets and tovolatilize the impurities. The very high temperature involved in thisoperation partially melts the iron oxide which, when it cools, rapidlyforms a glass-like material which fuses the pellets together. Not onlyis the resultant mass difficult to reduce, but the recovery ofimpuritiesis relatively ineffective.

I have invented a process for recovering iron from steel mill dust whichovercomes the problems involved in attempts of the prior art to recoverthe iron. My process produces hard pellets of partially reduced ironwith an iron oxide surface, which pellets are not fused together. Myprocess permits effective removal of impurities and recovery of the sameas oxides. Particularly, my process permits removal of substantially allof the zinc from the dust and collection of the same as a zinc oxide. Myprocess not only permits efficient recovery of what formerly was wastematerial, but itreduces contamination of the atmosphere in an economicalmanner.

BRIEF DESCRIPTION OF THE INVENTION One object of my invention is toprovide a method for recovering both iron and impurities from steel milldust which overcomes the difficulties involved in recovery attempts ofthe prior art.

Another object of my invention is to provide a method for recoveringiron from steel mill dust in the form of hard, unfused pellets of iron.

Another object of my invention is to provide a method of removing andrecovering impurities such as zinc in the form of an oxide thereof.

A still further object of my invention is to provide a method ofreclaiming what formerly was waste material in the steel-making process,while at the same time reducing contamination of the atmosphere.

Other and further objects of my invention will appear from the followingdescription.

In general, my invention contemplates the provision of a pelletreclamation process in which I form green pellets ofa mixture of steelfurnace dust and an internal fuel such as coke or coal. I feed the greenpellets over a layer of burnt pellets on a rotary hearth whichsuccessively conveys the pellets first through a drying zone in whichthe pellets are dried, then to the next zone in which the pellets areheated to a "temperature at which the coke begins to burn, then througha decontamination zone in which the pellet temperature is rapidly raisedto a degree at which the impurities such as zinc are first volatilizedand then carried off as oxides of the impurity. Finally, the pellets arecarried into a reoxidation and hardening zone wherein the temperaturethereof is further raised to a degree just below the melting point ofoxides of iron wherein the iron is reoxidized by the growth of grains ofan oxide of iron to form a hard dense shell on the pellets which are notfused together. Also the calcium oxide combines with ferric oxide toform calcium ferrites which also densify the surface layer.

BRIEF DESCRIPTION OF Til-IE DRAWINGS In the accompanying drawings whichform part of the instant specification and which are to be read inconjunction therewith and in which like reference numerals are used toindicate like parts in the various views:

FIG. 1 is a block diagram illustrating the various steps which areperformed in the practice of my pellet reclamation process.

FIG. 2 is a schematic view illustrating the relationship among thevarious pieces of apparatus which are employed in practicing my process.

FIG. 3 is a plan view with parts shown schematically of a typical plantwhich may be used to carry out my process.

FIG. 4 is a front elevation of the plant shown in FIG. 3.

FIG. 5 is a side elevation of the plant shown in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings,the initial step or steps in carrying out my process are indicated bythe block 10 in FIG. 1. These initial steps are the collection anddrying of the dust from the mill furnace which may be either the BOF orthe OH type. Clarifiers l2 deliver a slurry of dust to an underflow pump14, which feeds the slurry containing about 40 percent solids to a spraydrier I6 of any suitable type known to the art. I have indicated theflow ofthe slurry from the pump 14 to the drier 16 by the broken line 18in FIG. 2. I supply hot drying air to the drier as indicated by thedot-dash line 20 in FIG. 2. As will be explained more fully hereinafter,I obtain the air supplied to the drier 16 from a rotary hearth to bedescribed. Dust-laden air from the drier 16 passes through a bagcollector 26 of any suitable type known to the art, from which it iscarried by a fan 28 to a stack 30. The dust is delivered by the bagcollector 26 to bin 34. Bin 34 feeds a measured amount of the collecteddust to the conveyor 24.

In practice of my method, the next step, indicated by the block 40 inFIG. l, is the addition of finely divided coke dust to the dust 22. Adry coke breeze, indicated by the dot-dash line 42 in FIG. 2, suppliesthe coke dust to a collector 44 having an exhaust air filter 46. A scale48 associated with conveyor 24 permits me to measure the amount of driedcoke dust added by collector 44 to the dust 22. Preferably, I add cokedust in an amount of about from 10 to 12% by weight of the mixture.

As is indicated by the block in FIG. 1, the next step in the practice ofmy process is mulling or mixing the dried coke dust with the furnacedust. In order to achieve that operation, conveyor 24 delivers the dustto a dry pan muller 52 of a type known in the art. Muller 52 deliversthe dust to a second conveyor 54.

As shown in FIGS. 3 to 5, the elements of my apparatus which have beenthus far described, as well as those to be described, are housed in asuitable building, indicated generally by the reference character 56,having a floor 58, a roof and a plurality of side walls 62. The meanswhich carries the mixture of dust and coke from the muller 52 maycomprise a bucket elevator 64 leading to the conveyor 54 which carriesthe material to a pelletizer 66 in which the step, indicated by block 68in FIG. 1, of pelletizing the material is carried out. This pelletizer66 may be of any suitable type known to the art.

The apparatus for carrying out my process further includes a rotaryoven, indicated generally by the reference character 70, comprising ahearth 72 enclosed by a hood 74 and adapted to be driven in thedirection of the arrow A in FIG. 3. The structural details of an ovensuch as I employ in my process are more fully shown and described inU.S. Pat. No. 3,050,298. The oven 70 includes an entry 76 through whichthe material to be treated is delivered for deposit on the hearth 72. Inthe particular use to which I put the oven 70, I divide the hood intorespective zones, indicated generally by the reference characters 78,79, 80, 82 and 83, by means of partitions 84, 86, 87 and 88. An inletduct 90 is supplied with preheated air to provide combustion air for theburners (not shown) in the wall of the hood. It will readily beappreciated that the burners in the respective zones and the airsupplied thereto may be so controlled as to provide the properconditions to be described below to which the pellets are subjected in amanner to be described. A combustion fan 93 supplies air through a heatexchanger 94 to the duct 90 which leads to a distributing duct 96 whichsupplies the various zones. A heated air fan supplies hot air throughexchanger 94.

I provide the respective zones 78, 80 and 82 with gas collectingadapters 98, 100, 102 and 103. A respective exhaust duct 104 isconnected to each of the headers 98, and 102.

In the next step of my process I form on the hearth 70 a base layer ofburnt pellets which performs the dual functions of protecting therefractory material of the hearth 70 and of protecting the green pelletsdeposited thereover in a manner to be described from the heat of thedirect contact with the hearth. For this purpose I provide a burntpellet bin 108 at a location which is prior to the beginning of zone 78so as to distribute the burnt pellet layer thereon. Preferably, ldeposit only a single layer of such pellets which may have a diameter ofabout one-half inch on the surface of the hearth 72.

A reciprocating conveyor 110 leading from the pelletizer 66 to the entry76 deposits green pellets on top of the layer of burnt pellets which hasbeen deposited on the hearth. Preferably, the layer of green pellets isabout one and one-half times the diameter of an individual pellet. Owingto the fact that the bed of green pellets is only one or at most twopellets deep, all are exposed to radiation from the walls and roof andthus are heated faster.

With the system in operation, hearth 72 rotates to carry the greenpellet layer supported on the burnt pellets from the entry 76 around thehood 74 successively through the drying zone 78, through the heatingzone 80, through the decontamination zone 82 and through the reoxidationand hardening zone 83 to a point just ahead of the entry 76 wherein thepellets are removed and deposited in the cooler 112.

I so control the supply of air and the gas burners in the various zonesas to heat the pellets as follows. In drying zone 78 or the first zone,the pellets are heated for about 10 to about 15 minutes to a temperatureof 500F. to 600F. in the presence of a slightly oxidizing atmosphere.All of the heat in this zone comes through the hearth layer pellets fromthe hearth. Since, in the course of the drying step, air is not passedthrough the bed of pellets, they can be dried at a higher temperaturethan would be possible otherwise.

In the second zone, the temperature is raised to 1500F. to l600F. in thepresence of a slightly oxidizing atmosphere. The heat in this zone issupplied primarily by combustion of gas in the furnace hood. The pelletsare in this zone for about 7 to 10 minutes. In this zone the temperatureof the pellets is raised to a degree at which the internal fuel coke inthe pellets begins to burn but which temperature is lower than that at Iwhich the impurities begin to volatilize.

In the third zone, the temperature is raised rapidly to about 2000F. to2IO0F. and is held for about 7 to 15 minutes in a slightly reducing oroxygen poor atmosphere. At this temperature carbon has a greateraffinity for oxygen than does either iron or zinc, so that it takesoxygen both from the oxides of zinc and lead and the oxides of iron. Thezinc in the pellets is both in the form of zinc oxide and zinc ferrite.The former material reduces at approximately 1700F. while the latterrequires a temperature of about 2000F. to reduce. At the same time, thecarbon reduces the iron oxide toward metallic iron with the result thatthe iron oxides end up in this zone as FeO and Fe. The zinc, lead andsulfur all vaporize. The sulfur goes off with the flue gases. The leadfumes off as a part of the zinc vapor under the action of the exhaustapplied to header 100. These elements reoxidize above the bed and arecarried away through duct 104 to a bag collector 114 through whichexhaust gases are drawn by a fan 116, the exhaust of which is connectedto a stack 118. Any suitable means such, for example, as a pneumaticpump 120, carries the zinc oxide from the collector 114 to a bin 122having an exhaust air filter 124. In this manner the zinc oxide isreclaimed. The zinc oxide may also be granulated and dried so it can behandled and transported easier.

After the impurities are removed in the manner just described, thepellets are carried on through the fourth or reoxidation and hardeningzone 83. In this last zone the partially reduced iron is heated to atemperature of about 2300F. to 2450F. Air is introduced into this regionto oxidize the surface of the pellets and to cause the pellets to shrinkand thus harden. I so control the rate and temperature of the air addedto maintain the pellet at the desired temperature. It is important tonote that I so select this temperature as to be just below the meltingpoint of the oxide. I maintain this temperature for a period of about 7to l5 minutes to cause the pellets to gradually shrink and thus to forma hard bonded pellet. In this respect, my process differs from sinteringprocesses of the prior art; that is, in the sintering processes thepellets are subjected to a very high temperature which melts the ironoxide so that when it cools rapidly it forms a glass-like material whichis brittle and hard to reduce. In my process, no melting of the pellettakes place. The dense outer shell of my pellet is composed ofa thinlayer of hematite at the surface followed by a dense mixture ofmagnetite and calcium ferrite with most of the pores filled with asilicate slag phase. This surface zone is very dense and inhibitsoxidation of the core. The center of the pellet is made up of a porousFeO zone and a FeO, metallic Fe core.

I have indicated the respective steps of depositing the green pellets onthe base layer and of passing the pellets through the respective zonesby the blocks 126, 127, 128, 130 and 132 in FIG. 1. When the pelletsreach the end of the reoxidizing zone 83, they are removed to the cooler112. A fan 134 supplies cooling air to the pellet cooler 112 to cool thepellets in the step indicated by block 1136 in FIG. 1. I take the heatedair from cooler 112 and supply it with the air from zones 78 and 83 tothe drier 16.

A vibrating feeder 138 located below the cooler 112 receives pelletsfrom the cooler and delivers them to a vertical bucket elevator 148which delivers the cooled pellets to a screen 142. A reciprocatingconveyor 144 is so operated as to deliver a portion of the pellets fromscreen 142 to bin 108. The remaining pellets are received by a conveyor146 which delivers them to a pellet bin 148. It will be appreciated thatthe pellets received by conveyor 144 and delivered to bin 108 form thebase layer of burnt pellets on the hearth 72.

In practice of my pellet reclamation method, the slurry fed by pump 14to drier 16 contains about 40% solids, including oxides of lead and ironsulfates and oxides of iron. The heated air supplied to the drier 16from the initial heating zone 78 and from the reoxidizing zone 83 andfrom the cooler 112 removes water from the slurry so that particles ofdust 22 are fed directly by the drier to the bag collector 26 and fromthe dust bin 34. Finely divided coke from the coke collector 44 is mixedwith the steel furnace dust in muller 52 and is pelletized in pelletizer66.

Bin 108 delivers a base layer of burnt pellets to hearth 72 and conveyor118 spreads a layer of green pellets over the base layer of burntpellets. Hearth 72 carries the pellets first through the initial dryingzone 78 and then through a heating zone 83 in which the pellets areheated to a temperature which is sufficient to begin oxidation of theinternal coke but which is lower than the temperature at which theimpurity oxides are reduced. After the initial heating, the pellets arecarried through the decontamination zone 82 in which they are heatedrapidly to a temperature sufficient to reduce the impurity oxides and tobegin reducing the iron oxides. The impurities are taken away from thehearth in metallic form, reoxidized above the hearth and are carriedaway and collected in bag collector 114. Following removal of theimpurities, the pellets are carried into the reoxidation and hardeningzone 83 to which hot air is supplied so as to reoxidize the surfacewithout at the same time raising the pellets to a temperature above themelting point of the oxides. The pellets are held in this last zone fora period of time which is sufficient to permit shrinkage and hardeningso as to form the hard surface skin described hereinabove.

It will be seen that I have accomplished the objects of my invention. Ihave provided a pellet reclamation process which overcomes the defectsof reclamation processes of the prior art. My process forms hard ironoxide pellets which are not fused. It permits the formation of suchpellets while at the same time effectively removing and recoveringimpurities. Thus my process not only permits recovery both of iron andof impurities in furnace dust, but it also greatly reduces theatmosphere pollution problems.

It will be understood that certain features and subcombinations are ofutility and may be employed with out reference to other features andsubcombinations. This is contemplated by and is within the scope of myclaims. It is further obvious that various changes may be made indetails within the scope of my claims without departing from the spiritof my invention. It is, therefore, to be understood that my invention isnot to be limited to the specific details shown and described.

Having thus described my invention, what I claim is:

l. A method of recovering iron from steel furnace dust containing oxidesof iron and zinc including the steps of passing wet dust through adrier, supplying said drier with hot air from the hood of a hearth todry said dust, mixing said dust with about 5 to 7 percent by weight offinely divided coke, forming green pellets of said mixture, burning someof said pellets, depositing a layer of said burnt pellets on a hearth,dividing said hood into a drying zone, a heating zone, a decontaminationzone and a reoxidation and hardening zone, said zones being separate anddistinct "from each other, depositing said green pellets on said layerof burnt pellets, passing said pellets while on said! hearthsuccessively through said drying zone and heating zone and saiddecontamination zone and said reoxidizing and hardening zone, dryingsaid pellets in said drying zone at a temperature of about 500F. toabout. 600F., heating said pellets in said heating zone to a temperatureof about l500F. to 1600F., then rapidly heating said pellets in saiddecontamination zone to an elevated temperature of about 2000F. to2100F. and maintaining said pellets at said elevated temperature in saiddecontamination zone for sufficient time to volatilize said zinc,removing said volatilized zinc from said decontamination zone as anoxide of zinc, collecting said zinc ox ide, heating said pellets in saidreoxidation zone to a higher temperature of from about 2300F. to about2450F., maintaining said pellets at said higher temperature in saidreoxidation zone for sufficient time to cause grains of hematite to growto form a hard bonded pellet and removing said pellets from said hearthafter passage through said reoxidation zone.

2. A method as in claim 1 in which said heating step at l500F. to l600F.is for about 10 to l5 minutes.

3. A method as in claim 1 in which said drying step comprises heatingsaid pellets for about 10 to 15 minutes at a temperature of about 500F.to 600F.

4. A method as in claim 1 in which said rapidly heating step comprisesheating said pellets to a temperature of about 2000F. and holding saidpellets at said temperature for about 7 to about 15 minutes.

5. A method as in claim 1 in which said further heating step comprisesheating said pellets to a temperature of about 2300F. to about 2450F.and maintaining said pellets at said temperature for about 7 to about 15min- 7. A method as in claim 1 including the steps of reutes to formsaid hematite grains. moving said pellets from said hearth after passage6. A method as in claim 1 including the steps of rethrough saidreoxidizing zone and cooling said removed moving said wet dust from aslurry and drying said dust pellets. before forming said mixture.

2. A method as in claim 1 in which said heating step at 1500*F. to1600*F. is for about 10 to 15 minutes.
 3. A method as in claim 1 inwhich said drying step comprises heating said pellets for about 10 to 15minutes at a temperature of about 500*F. to 600*F.
 4. A method as inclaim 1 in which said rapidly heating step comprises heating saidpellets to a temperature of about 2000*F. and holding said pellets atsaid temperature for about 7 to about 15 minutes.
 5. A method as inclaim 1 in which said further heating step comprises heating saidpellets to a temperature of about 2300*F. to about 2450*F. andmaintaining said pellets at said temperature for about 7 to about 15minutes to form said hematite grains.
 6. A method as in claim 1including the steps of removing said wet dust from a slurry and dryingsaid dust before forming said mixture.
 7. A method as in claim 1including the steps of removing said pellets from said hearth afterpassage through said reoxidizing zone and cooling said removed pellets.